1. Field of the Invention
The present invention relates to a bar code scanner.
2. Description of the Prior Art
Recently, for processing by computer the large volume of commodity data in each of the manufacture, storage, distribution, sales phases, etc. of commodities, identification number of a commodity has become marked on the container, package or the like of the commodity.
The bar code is generally a combination of wide and narrow bars with which figures, characters or other information are coded. The bar code is provided on the container, package or the like as printed directly there or attached in the form of a bar code label.
A bar code scanner scans the bar code optically and converts it into an electrical signal which will be sent as input to the computer for processing.
The bar code scanners are available in various types; the basic principle of all the types is projecting light to the bar code and identifying the width of each bar based on the quantity of reflected light from the bar. This bar code scanning can be done by either of the following two methods. One of the method is such that a fine beam spot is scanned perpendicularly to code bars and reflected from each bar is scanned one after another by a single photodetector element. The other method is to projecting light evenly to the bar code surface and scanning the reflected lights from the code bars simultaneously by a photodetector or CCD (character coupled device) image sensor composed of a plural photodetector elements provided correspondingly to the code bars. The present invention adopts the latter one of the above-mentioned methods.
The bar code scanner is an apparatus to receive the light reflected from or penetrating through the bar code label and recognize the data carried by the bar code, and it consists of a scan head section and a decoder section incorporating a micro processor to decode the contents of the bar code.
More particularly, bar code scanners of the above-mentioned type comprise a light source to project light evenly to the bar code surface, a lens system to converge the scattered or reflected lights on the bar code surface, a photoelectric transducer (photodetector) element to convert the light collected by the above lens system into an electrical signal, a decoder which shapes the electrical signal obtained by the above-mentioned photoelectric conversion and analyzes information from the figures and letters represented by the bar code, and an interface unit to interface the data from said decoder to an external unit (host computer). Conventionally, the above-mentioned light source, lens system and photoelectric transducer element are contained as the scan head in one enclosure while said decoder, interface unit, a power circuit, etc. are housed in another enclosure separate from the scan head. These two enclosures are connected by a cable.
A prior-art example will be described below with reference to FIGS. 10(A) and (B). As seen from these figures, a light source 100 consisting of an array of lamps or LEDs (light emitting diode) is provided at an angle with respect to an optical axis of light reception 101. The light (optical axis of light projection) 102 from the light source 100 is projected evenly onto the surface of a bar code label 103 after passing through an opening 104. The optical axis of light reception 101, namely, the reflected light from the bar code label 103 is subject to a predetermined optical process through an optical system 105, and then it is incident, through an optical path 106, upon a photodetector 107. The phtodetector 107 is a linear array of a required number of photodetector elements for the number of bars of the bar code. Each photodector elements send electrical signals obtained by photoelectric conversion in parallel to a signal processor 108.
As apparent from the above, the size of the bar code to which the bar code scanner is applicable is limited by the length of the opening 104 of the bar code scanner. The light source 100 for light projection is installed just inside the opening 104 at the end of the bar code scanner at an angle with respect to the optical axis 101 of light reception. This light source is a linear array of plural high-brightness, high-directionality LEDs disposed as regularly spaced in line in the same direction as that in which the bars of the bar code label 103 are disposed. In such light source 100, projected beams of light are overlapped on each other in a narrow range since each of the LEDs has a high directionality. The luminance distribution on the illuminated surface of the bar code label 103 is nonuniform since the brightness on the optical axis of light projection of each LED is high while that between adjoining optical axis of light projection is low. To eliminate such nonuniformity of light projection in order to attain an even luminance distribution, it has been proposed heretofore to use LEDs of wide directionality or to use a cylindrical lens or the like before the LEDs. However, such high directionality LED should be of a high brightness, and so it consumes so much electric power. Also the use of the cylindrical lens led to a complicated structure of the bar code scanner and also to correspondingly high costs.
The optical geometrical relation between the light source 100 of the bar code scanner and the bar code label 103 illuminated by the light source 100, shown in FIGS. 10(A) and (B) is shown in FIGS. 11(A) and (B). The bar code label 103 has a bar code printed on the surface thereof as shown. The light source 100 is a linear array of plural LEDs 109. Each of these LEDs 109 has a high brightness and directionality, and emits a fine beam of light. The light beams from these LEDs 109 form together a composite beam of light 110. The area 111 of the bar code label 103 onto which the composite light beam 110 is projected is indicated with a dash line. The illuminated area 111 can be extended to a desired length by disposing a required number of LEDs 109 in the disposed direction (indicated with W) of the bars of bar code. However, the lengthwise direction (indicated with H) of the bars of bar code is limited by the beam width of one LED 109. Therefore, when the bar code is wholly in the illuminated area 111 by directing the scan head correctly toward the bar code label 103 as shown in FIG. 11(A), the reflected light 112 from all the LEDs which are reflected by the bar code label 103 coincide with the optical axes of light reception 101 of the scan head, and thus the reflected light beams are picked up into the scan head through the opening 104.
However, if the light source 100 is inclined somewhat leftwise as shown in FIG. 11(B), the area 111 lighted by the composite beam of light 110 is also inclined as indicated with the dash line. As the result, the luminance of the reflected light 112 from the bar code is low, and in the worst case, the reflected light 112 is so small, like the beam at the lower left portion of FIG. 11(B), as not to be detectable by the photodetector element.
FIG. 12 is a circuit diagram of the light source 100 of the prior-art scan head, the reference numeral 113 indicating a current limiting resistor. These resistors 113 are used to supply each of as many LEDs 109 as them, being arranged as regularly spaced, with a same current so that the LEDs will always emit light.
The prior-art bar code scanners generally include stationary types and hand-held types, which are used in their respective manners. The hand-held type scan head is so designed as to start a scan with the head brought as held by hand near the bar code attached or printed on the commodity. The methods of bar code scanning include two kinds: In one method, the scan is effected by operating a switch to let the LEDs 109 in the scan head emit light with the opening of the scan head brought near the bar code label 103. In the other method, the scan is done with the LEDs always in the state of emitting light. The former method needs operation of the LED on/off switch every time the scan head is brought near the bar code and thus will cause the operator to be very tired in case a large quantity of goods is to be inspected, while the latter method will not cause the operator to be tired as in the former method, but it will consume much electric power since the LEDs are always in the state of light emission.